I'm using the plugin Video Thumbnails to fetch thumbnails from a video url specified in a custom field.
It works great in the bakend, but on my site the posts are submitted on the front end with this code:

Assuming you've already modified the plugin for the custom field, all I think you'll need to do is add a line right after update_post_meta( $the_post_id, '_videoembed', $video_url); with get_video_thumbnail($the_post_id);